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Tesla's electric car offers a quiet, powerful ride. But unless it comes up with a cheaper, stronger battery, the stock could turn out to be a lemon.

Google "Iron Man, Tony Stark" and within the first few results you'll find Websites likening that superhero to Elon Musk, 41, the entrepreneur behind PayPal, the rocket maker SpaceX, and the electric-car sensation Tesla Motors. The comparison's apt. Musk is smart and stylish, and he fights planetary threats like global warming by creating spacecraft and zero-emissions cars that shame NASA and the auto giants. He's as rich as Iron Man's armored billionaire, too. Tesla shares rocketed this year from $35 to $115 -- lifting the Palo Alto auto start-up to a market value of $14 billion at May's end -- before...